Immobilized Co/Rh heterobimetallic nanoparticle-catalyzed Pauson-Khand-type reaction

Co/Rh heterobimetallic nanoparticles were prepared from cobalt-rhodium carbonyl clusters [Co2Rh2(CO)(12) and Co3Rh(CO)(12)] and immobilized on charcoal. HR-TEM revealed that the size of the heterobimetallic nanoparticles was ca. 2 nm and ICP-AES analysis showed a 2:2 and a 3: 1 cobalt-rhodium stoichiometry (Co2Rh2 and Co3Rh1) in the heterobimetallic nanoparticles. The Co/Rh heterobimetallic nanoparticles immobilized on charcoal were used as a catalyst in the Pauson-Khand-type reaction under 1 atm of CO. The catalytic reactivity was highly dependent upon the ratio of Co:Rh with the highest reactivity being observed when the ratio was 2:2 (Co2Rh2). The Co2Rh2 immobilized catalyst is quite an effective catalyst for intra- and intermolecular Pauson-Khand-type reactions. When the immobilized Co2Rh2 catalyst was used as a catalyst in the Pauson-Khand-type reaction in the presence of an aldehyde instead of carbon monoxide, the catalytic system was highly efficient. When the reaction was carried out in the presence of chiral diphosphines, ee values up to 87% were observed. The catalytic system can be reused at least five times in the presence of chiral diphosphines without loss of catalytic activity and enantioselectivity. The addition of Hg(0), a known heterogeneous catalyst poison, completely inhibits further catalysis. Thus, an environmentally friendly and sustainable process was developed.